In a conventional extrusion press, provision is made for an end platen and a main cylinder device which are coupled by tie rods. At the end platen side, a container in which a billet is loaded is arranged across a die, while at a main cylinder device side, a stem is attached to a cross head which is driven together with a ram which is inserted into and withdrawn from this cylinder. Further, the stem is pushed under pressure toward the billet which is loaded in the container by the extrusion force of the main cylinder device whereby a product of a predetermined shape is extruded from the die.
In such an extrusion press, in the extrusion process, the length of the billet inside the container gradually becomes shorter, so it is usual that the extrusion action force becomes larger at the time of start of the extrusion process than at the time of end of the extrusion process. That is, even if the extrusion resistance of the die (required extrusion force) is constant, the frictional force between the container and the billet becomes smaller along with the decrease in the length of the billet, so overall the extrusion action force gradually ends up falling.
The change in the extrusion action force is due to the decrease in the frictional force between the billet and the inside walls of the container along with the billet length, but such a decrease in the extrusion action force causes a decrease in the container seal force with respect to the die. When the force falls below the seal force which prevents breakout of the billet, the billet ends up breaking out from the seal surface of the container and die in a so-called “blooming phenomenon”. This blooming phenomenon causes the product yield to drop and lowers the operating efficiency of the extrusion press due to the work of removing the residue from the seal surface.
For this reason, when the extrusion action force falls and becomes less than the seal force which prevents break out of the billet, a means which compensates for the drop in the extrusion action force which imparts the container seal force between the container and the die may be provided at the cross head so as to avoid break out of the billet. (See PLT 1.)
In this regard, since, in the above conventional type of extrusion press, to prevent break out of the billet, a means for compensating for the drop in extrusion action force is provided at the cross head, this led to enlargement of the facility and complication of the control and further resulted in a rise in the cost of manufacture of the facility.